Which of the following correctly describes significant figures?

Significant figures are used to express the precision of a measurement and reflect the certainty of the data. The correct choice states that all digits that are directly measured, along with one estimated digit, count as significant figures. This means that every non-zero digit is considered significant, as well as any zeros between non-zero digits. The estimated digit represents the precision of the measurement, indicating that it is not exact.

In this context, options that suggest only whole numbers, only the last digit, or all digits including leading zeros do not accurately represent the concept of significant figures. Whole numbers alone neglect the importance of decimal precision, while only considering the last digit disregards all preceding measurements that contribute to the value. Lastly, treating leading zeros as significant figures is incorrect since they do not contribute to the precision of a measurement as they merely serve as placeholders. Thus, the identification of significant figures is vital for maintaining accuracy in scientific measurements and calculations.